1. Field of the Invention
The present invention relates to an apparatus wherein a DC (direct current) motor is used to provide the driving force for performing mechanical operations, and wherein stabilization of the rotational speed of the DC motor and control of cumulative rotation numbers of the DC motor are required. More particularly, the present invention relates to a rotation detecting apparatus and a rotation control apparatus for a DC motor wherein rotational operations of the rotor of a DC motor are controlled by detecting at least one of the rotational direction, the rotational speed, the cumulative rotation number, and the rotational position of the rotor.
2. Discussion of the Background
A brush-use DC motor is often used as a driving force for mechanical operations in a camera, such as, for example, zooming operations wherein photographic lenses including a zoom lens are zoomed, focusing operations wherein at least one of a photographic lens and an imaging device is moved along an optic axis of the photographic lens for focusing based on the information of a distance from an object to an image focusing point, and film feeding operations wherein a photographic film is wound and rewound.
In the brush-use DC motor, plural fixed magnetic poles are formed in a stator employing a permanent magnet, etc. A DC drive current is switched corresponding to a rotation angle of a rotor and is applied to plural rotor coils forming plural magnetic poles of the rotor through a commutator which rotates together with the rotor and through a brush which is in sliding contact with the commutator. Thereby, the rotor rotates.
There are, for example, five types of apparatuses using a motor as a driving force: (1) uni-directional rotations of the motor are used, and a rotational speed of the motor is required to be kept constant; (2) uni-directional rotations of the motor are used, and a cumulative rotation number of the motor, that is, a total driving amount of the motor, is required to be controlled; (3) bi-directional rotations of the motor (i.e., a forward rotation and a reverse rotation) are used, and a rotational speed only on uni-directional rotations of the motor is required to be kept constant; (4) bi-directional rotations of the motor are used, and each rotational speed on bi-directional rotations of the motor is required to be kept constant; and (5) bi-directional rotations of the motor are used, and a cumulative rotation number on unidirectional rotations of the motor is required to be controlled.
With regard to a rotation control method of a motor in an apparatus, there are, for example, two types of apparatuses according to their uses and operation environmental conditions; (1) a rotational speed of the motor is controlled by changing a drive voltage for driving the motor, and (2) a rotational speed of the motor is controlled by a chopping control wherein a drive voltage is intermittently applied to the motor.
As an example of the above-described brush-use DC motor, FIG. 35 illustrates a three-pole motor. In the three-pole motor, electricity is fed to a commutator CM0 which is in sliding contact with a pair of electrode brushes B01 and B02 from a DC drive power supply E0 through the paired electrode brushes B01 and B02. The paired electrode brushes B01 and B02 are brought into contact with the commutator CM0 on rotation angle positions which differ by 180xc2x0. The commutator CM0 includes three pieces which form a cylindrical surface and rotates together with a rotor of the DC motor. The three pieces of the commutator CM0 are separated at an equally angled interval of about 120xc2x0. Three rotor coils are connected to each other between the adjacent pieces of the commutator CM0, and thereby three rotor magnetic poles are formed therebetween. The polarity of these rotor magnetic poles varies depending on the contact state of each piece of the commutator CM0 and the electrode brushes B01 and B02 which changes corresponding to the rotation angle of the rotor. Thereby, a rotation driving force is generated between, for example, a pair of stator magnetic poles of a permanent magnet at the side of a stator (not shown).
With the rotation of the rotor, respective rotor magnetic poles oppose to respective stator magnetic poles in order, and the contact state of each piece of the commutator CM0 and the electrode brushes B01 and B02 changes. Thus, by the variance of the polarity of each rotor magnetic pole in order, the rotor continually rotates.
Specifically, when a voltage is applied to the paired electrode brushes B01 and B02 from the power supply E0, the current flows from one of the electrode brushes B01 and B02 to another through the rotor coils. The magnetic field is generated by the rotor coils, and thereby the rotor magnetic poles are formed. By the action of the magnetic field generated by the rotor coils and the magnetic field generated by the stator magnetic poles, the rotor rotates.
As a method of detecting the rotation of the above-described motor, a rotary encoder method is known. Specifically, in the rotary encoder method, a rotation slit disk having slits on its circumferential surface is provided on a rotation output shaft of the motor or in a power transmission mechanism rotated by the rotation output shaft. The rotation of the motor is detected by the method of detecting the slits on the circumferential surface of the rotation slit disk with a photointerrupter. Although the rotary encoder method allows an accurate detection of the rotation of the motor, space and cost for the rotary encoder constructed by the rotation slit disk, the photointerrupter, etc. are inevitably increased.
Further, another method of detecting the rotation of the motor from the drive voltage ripple of the motor is described referring to FIGS. 36 and 37. In FIG. 36, a resistor R0 is connected in series to electrode brushes B01 and B02 in a power supplying line for supplying the motor drive current to the electrode brushes B01 and B02 from a drive power supply E0, and the voltage between both terminals of the resistor R0 is detected. In such a way, the ripple waveform of 60xc2x0-period of the drive current as illustrated in FIG. 37 is obtained.
Because the ripple waveform corresponds to the rotation angle position of a rotor, the pulse signal corresponding to the rotation angle position can be obtained by suitably rectifying (shaping) the ripple waveform. Although this another rotation detecting method is advantageous in cost and space, detection errors due to noise cause inaccuracies. Thus, this rotation detecting method is disadvantageous.
Japanese Laid-open patent publication No. 4-127864 describes another method for detecting a rotational speed of a DC motor wherein a rotation detecting brush is provided in addition to a pair of electrode brushes. The rotation detecting brush is brought into sliding contact with a commutator so as to extract a voltage applied to the commutator. The rotational speed of the DC motor is detected based on the signal generated by the rotation detecting brush.
Further, Japanese Laid-open patent publication No. 4-127864 describes a DC motor control circuit illustrated in FIG. 38. Referring to FIG. 38, a rotation detecting brush BD0 is provided to a motor M0 in addition to a pair of electrode brushes B01 and B02. The rotation detecting brush BD0 is connected to a differentiating circuit 101, a time constant reset circuit 102, and a time constant circuit 103 in order. In a comparator 105, the voltage of the output signal from the time constant circuit 103 is applied to a non-inversion input terminal (i.e., + side) of the comparator 105, and the voltage of the output signal from a reference voltage generating device 104 is applied to an inversion input terminal (i.e., xe2x88x92 side) of the comparator 105. The output signal from the comparator 105 is connected to one terminal of exciting coils of a relay 107 through a diode 106. Another terminal of the exciting coils of the relay 107 is connected to one terminal of a drive power supply E0. The pair of electrode brushes B01 and B02 is connected to the drive power supply E0 via a contact 107a of the relay 107.
One terminal of the exciting coils of the relay 107 is connected to a collector of a transistor 109a of a motor starting circuit 109 via a diode 108. The motor starting signal is applied to a base of the transistor 109a via a resistor 109b. A resistor 109c is connected between the base and an emitter of the transistor 109a. The emitter of the transistor 109a is connected to another terminal of the drive power supply E0.
FIG. 39 is a diagram illustrating waveforms of a motor starting signal input to the motor starting circuit 109, a rotation detecting signal SA0 of the rotation detecting brush BD0, an output signal SB0 from the differentiating circuit 101, an output signal SC0 from the time constant circuit 103, an output signal SD0 from the comparator 105, an operation (on/off) signal of the relay 107, and a supply signal applied to a motor M0 from a drive power supply E0.
When the transistor 109a of the motor starting circuit 109 is turned on by the motor starting signal, the relay 107 is turned on and the contact 107a is closed. Thereby, the electric power is supplied to the motor M0 through the electrode brushes B01 and B02, and the motor M0 starts rotating.
With the rotation of the motor M0, pulse train SA0 is output from the rotation detecting brush BD0 and is differentiated in the differentiating circuit 101. Then, signal SB0 which is synchronized in the leading edge of each pulse is applied to the time constant reset circuit 102. The time constant reset circuit 102 is synchronized in the signal SB0, and resets the time constant circuit 103. Then, signal SC0 is output from the time constant circuit 103 as illustrated in FIG. 39.
In the normal state in which the motor M0 rotates at a usual rotational speed, the voltage of the output signal SC0 from the time constant circuit 103 does not exceed the reference voltage applied from the reference voltage generating device 104. In this state, output signal SD0 from the comparator 105 is in an xe2x80x9cLxe2x80x9d (low) level, and the relay 107 is excited and keeps an ON condition. Thereby, the supply of electricity to the motor M0 is maintained.
However, when the rotational speed of the motor M0 is lowered by overloads, etc., the voltage of the output signal SC0 from the time constant circuit 103 exceeds the reference voltage. Thereby, the output signal SD0 from the comparator 105 becomes a xe2x80x9cHxe2x80x9d (high) level, and the exciting current does not flow through the relay 107. Thereby, the relay 107 is turned off, and the contact 107a is opened. As a result, the supply of electricity to the motor M0 is stopped.
Thus, in the above-described DC motor control circuit, the lowering of the rotational speed of the motor M0 is detected, and excessive current is prevented from flowing in the motor M0 by stopping the DC motor M0.
Japanese Laid-open patent publication No. 4-127864 describes a DC motor control circuit wherein only when the rotational speed of the motor M0 is lower than the certain rotational speed is the relay 107 turned off.
DC motor rotation control apparatuses which detect and control the rotational speed, the rotational position, the cumulative rotation number, and the rotational direction of the DC motor with high accuracy are heretofore not known in the art.
According to one aspect of the present invention, a direct current motor rotation detecting apparatus and a direct current motor rotation control apparatus which are configured to detect and control at least one of a rotational direction, a rotational speed, a rotational position, and a cumulative rotation number of a rotor of a direct current motor, include at least one rotation detecting brush configured to detect a rotation of the rotor, a differentiating circuit configured to differentiate a voltage obtained through the at least one rotation detecting brush, and a pulse generator configured to be triggered by the differentiating circuit at a time interval proportional to a rotational speed of the direct current motor and to generate pulses having a predetermined pulse width.
The present invention also includes a method for detecting at least one of a rotational direction, a rotational speed, a rotational position, and a cumulative rotation number of a rotor of a direct current motor. The method includes detecting a rotation of the rotor using at least one rotation detecting brush, differentiating a voltage obtained through the at least one rotation detecting brush, triggering a pulse generator by a differentiating device, and generating pulses having a predetermined pulse width.